Thin film transistor-liquid crystal displays (TFT-LCD) are widely used for flat panel display devices in many applications. The TFT-LCD is an especially useful liquid crystal display because the TFT-LCD is capable of a large contrast ratio and may be readily adapted for color displays. In addition, because large screens can be made without reducing image quality, the TFT-LCD is expected to be applied to high definition TV and other fields.
As is well known to those having skill in the art, a TFT-LCD includes a plurality of liquid crystal cells and a plurality of thin film transistors, a respective pair of which is serially connected between a common electrode and a plurality of drivers.
Unfortunately, crosstalk may be produced in a TFT-LCD. For example, the crosstalk from a white or black area which is displayed may influence the surrounding cells to display different gray voltage levels than is intended, thereby producing a blurred image.
There are two general types of crosstalk: vertical crosstalk and horizontal crosstalk. Vertical crosstalk may be generated when a thin film transistor is not fully turned off because the unwanted gray voltage which is applied by a data line connected to the source of the thin film transistor, is transferred to the liquid crystal cell through the drain terminal of the thin film transistor. Horizontal crosstalk may be generated when a desired gray voltage is not applied to a liquid crystal cell because of potential differences between two adjoining liquid crystal cells which are connected to the common electrode. The potential difference may cause current to flow to adjacent liquid crystal cells rather than only to a selected liquid crystal cell.
FIGS. 8, 9A, 9B, 10A and 10B graphically illustrate crosstalk in a TFT-LCD. FIG. 8 is an equivalent circuit of a TFT-LCD which omits the thin film transistor connected to each liquid crystal cell. As shown, the voltage Vcom is applied to a common electrode having an internal resistance Rcom. Clc1, Clc2 . . . are the associated capacitances of a liquid crystal cell.
In a conventional TFT-LCD as illustrated in FIG. 8, the voltage applied to the liquid crystal cell is the difference between the common electrode voltage Vcom and the gray voltage which is applied via the thin film transistor. The brightness of a cell is determined based upon the voltage which is applied to the liquid crystal cell.
Generally, when white is displayed, the voltage potential difference between the common electrode and the gray voltage terminal of a liquid crystal cell is at a minimum and when black is displayed, the potential difference between the common electrode and the gray voltage terminal is a maximum. Therefore, the amount of electric charge in the liquid crystal cell is generally a minimum for white and a maximum for black. Accordingly, the amount of current flowing in the common electrode is generally a minimum for white and a maximum for black. Thus, the amount of current which flows in the common electrode changes based on the displayed level.
FIG. 9A is a waveform illustrating the voltage which is applied to the common electrode. FIG. 9B is an output waveform of the voltage at the common electrode. As shown in FIG. 9B, when white is displayed, there is generally no distortion in the common electrode voltage waveform. However, when black is displayed, distortion generally occurs in the common electrode voltage waveform. This distortion is generally attributed to the internal resistance of the panel. Due to this internal resistance, the amount of current flowing in the common electrode is greater when black is displayed, and the voltage drop difference influences the common electrode waveform.
FIG. 10A illustrates a common electrode voltage waveform which is applied to two terminals of a liquid crystal cell for white, and Figure 10B illustrates the common electrode voltage waveform for black. As shown in FIG. 10A, the upper potential is a common electrode voltage and the lower potential is a gray voltage potential. In FIG. 10B, the upper potential is a common electrode voltage and the lower potential is a gray voltage.
As shown in FIGS. 10A and 10B, the areas A and B represent the total amount of charge in a liquid crystal cell. As also shown in FIGS. 10A and 10B, the common electrode voltage potential of a cell is different depending on whether white or black is displayed, due to the distortion of the common electrode voltage. Thus, the area A is generally different from the area B. The difference between the two areas can cause the difference between the gray display. Therefore, even though the same gray voltage level is applied to a liquid crystal cell, the display intensity in the cell is different based on the display in the surrounding cells.
It is known to reduce the above-identified differences in areas by reducing the resistance value of the panel (Rcom). Although this may reduce crosstalk, high performance applications of TFT-LCD displays may demand further cross-talk reductions. Moreover, as the size of the liquid crystal display panel increases, the resistance of the common line also tends to increase. Thus, the distortion generally increases in proportion to the increase in the panel size. Crosstalk therefore continues to be a problem in the TFT-LCD.